# std::ranges::copy_n, std::ranges::copy_n_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I, std::weakly_incrementable O > requires std::indirectly_copyable<I, O> constexpr copy_n_result<I, O> copy_n( I first, std::iter_difference_t<I> n, O result ); | (1) | (since C++20)
Helper type |  |
template< class I, class O > using copy_n_result = ranges::in_out_result<I, O>; | (2) | (since C++20)

1) Copies exactly n values from the range beginning at first to the range beginning at result by performing *(result + i) = *(first + i) for each integer in `[`​0​`,` n`)`. The behavior is undefined if result is within the range `[`first`,` first + n`)` (ranges::copy_backward might be used instead in this case).

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first | - | the beginning of the range of elements to copy from
n | - | number of the elements to copy
result | - | the beginning of the destination range

### Return value

ranges::copy_n_result{first + n, result + n} or more formally, a value of type ranges::in_out_result that contains an std::input_iterator iterator equals to ranges::next(first, n) and a std::weakly_incrementable iterator equals to ranges::next(result, n).

### Complexity

Exactly n assignments.

### Notes

In practice, implementations of `std::ranges::copy_n` may avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy `contiguous_iterator`. Alternatively, such copy acceleration can be injected during an optimization phase of a compiler.

When copying overlapping ranges, `std::ranges::copy_n` is appropriate when copying to the left (beginning of the destination range is outside the source range) while std::ranges::copy_backward is appropriate when copying to the right (end of the destination range is outside the source range).

### Possible implementation

    struct copy_n_fn
    {
        template<std::input_iterator I, std::weakly_incrementable O>
        requires std::indirectly_copyable<I, O>
        constexpr ranges::copy_n_result<I, O>
            operator()(I first, std::iter_difference_t<I> n, O result) const
        {
            for (std::iter_difference_t<I> i {}; i != n; ++i, ++first, ++result)
                *result = *first;
            return {std::move(first), std::move(result)};
        }
    };

    inline constexpr copy_n_fn copy_n {};

### Example

Run this code

    #include <algorithm>
    #include <iomanip>
    #include <iostream>
    #include <iterator>
    #include <string>
    #include <string_view>

    int main()
    {
        const std::string_view in {"ABCDEFGH"};
        std::string out;

        std::ranges::copy_n(in.begin(), 4, std::back_inserter(out));
        std::cout << std::quoted(out) << '\n';

        out = "abcdefgh";
        const auto res = std::ranges::copy_n(in.begin(), 5, out.begin());
        std::cout
            << "*(res.in): '" << *(res.in) << "', distance: "
            << std::distance(std::begin(in), res.in) << '\n'
            << "*(res.out): '" << *(res.out) << "', distance: "
            << std::distance(std::begin(out), res.out) << '\n';
    }

Output:

    "ABCD"
    *(res.in): 'F', distance: 5
    *(res.out): 'f', distance: 5

### See also

ranges::copyranges::copy_if (C++20)(C++20) | copies a range of elements to a new location (algorithm function object)
ranges::copy_backward (C++20) | copies a range of elements in backwards order (algorithm function object)
ranges::remove_copyranges::remove_copy_if (C++20)(C++20) | copies a range of elements omitting those that satisfy specific criteria (algorithm function object)
ranges::replace_copyranges::replace_copy_if (C++20)(C++20) | copies a range, replacing elements satisfying specific criteria with another value (algorithm function object)
ranges::reverse_copy (C++20) | creates a copy of a range that is reversed (algorithm function object)
ranges::rotate_copy (C++20) | copies and rotate a range of elements (algorithm function object)
ranges::unique_copy (C++20) | creates a copy of some range of elements that contains no consecutive duplicates (algorithm function object)
ranges::move (C++20) | moves a range of elements to a new location (algorithm function object)
ranges::move_backward (C++20) | moves a range of elements to a new location in backwards order (algorithm function object)
copy_n (C++11) | copies a number of elements to a new location (function template)

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